The present invention relates to elastomeric fuser members for electrostatographic printing apparatus. In particular it relates to compositions having improved thermal conductivity in fuser member applications.
In electrostatographic reproducing apparatus commonly used today a photoconductive insulating member is typically charged to a uniform potential and thereafter exposed to a light image of an original document to be reproduced. The exposure discharges the photoconductive insulating surface in exposed or background areas and creates an electrostatic latent image on the member which corresponds to the image contained within the original document. Alternatively, a light beam may be modulated and used to selectively discharge portions of the charged photoconductive surface to record the desired information thereon. Typically, such a system employs a laser beam. Subsequently, the electrostatic latent image on the photoconductive insulating surface is made visible by developing the image with developer powder referred to in the art as toner. Most development systems employ developer which comprises both charged carrier particles and charged toner particles which triboelectrically adhere to the carrier particles. During development the toner particles are attracted from the carrier particles by the charged pattern of the image areas of the photoconductive insulating area to form a powder image on the photoconductive area. This toner image may be subsequently transferred to a support surface such as copy paper to which it may be permanently affixed by heating or by the application of pressure. The present invention relates to the fusing of the toner image on a support.
The use of thermal energy for fixing toner images onto a support member is well known. In order to fuse electroscopic toner material onto a support surface permanently by heat, it is necessary to elevate the temperature of the toner material to a point at which the constituents of the toner material coalesce and become tacky. This heating causes the toner to flow to some extent into the fibers or pores of the support member. Thereafter, as the toner material cools, solidification of the toner material causes the toner material to be firmly bonded to the support.
Several approaches to thermal fusing of electroscopic toner images have been described in the prior art. These methods include providing the application of heat and pressure substantially concurrently by various means: a roll pair maintained in pressure contact; a flat or curved plate member in pressure contact with a roll; a belt member in pressure contact with a roll; and the like. Heat may be applied by heating one or both of the rolls, plate members or belt members. The fusing of the toner particles takes place when the proper combination of heat, pressure and contact time are provided. The balancing of these parameters to bring about the fusing of the toner particles is well known in the art, and they can be adjusted to suit particular machines or process conditions.
One approach to thermal fusing of toner material images onto the supporting substrate has been to pass the substrate with the unfused toner images thereon between a pair of opposed roller members at least one of which is internally heated. During operation of the fusing system of this type, the support member to which toner images are electrostatically adhered is moved through the nip formed between the rolls with the toner image contacting the fuser roll thereby to affect heating of the toner images within the nip. Typical of such fusing devices are two roll systems wherein the fusing roll is coated with an abhesive material, such as silicone rubber or other low surface energy elastomer as, for example, tetrafluoroethylene resin sold by E. I. DuPont de Nemours under the tradename Teflon. The silicone rubbers which can be used as the surface of the fuser member can be classified into three groups according to the vulcanization method and temperature, i.e. room temperature vulcanization silicone rubber hereinafter referred to as RTV silicone rubber, liquid injection moldable or extrudable silicone rubber, and high temperature vulcanization type silicone rubber, referred to as HTV rubber. All these silicone rubbers or elastomers are well known in the art and are commercially available.
During operation of a fusing system in which heat is applied to cause thermal fusing of the toner particles onto a support, both the toner image and the support are passed through a nip formed between the roll pair, plate or belt members. The concurrent transfer of heat and the application of pressure in the nip effects the fusing of the toner image onto the support. It is important in the fusing process that no offset of the toner particles from the support to the fuser member takes place during normal operations. Toner particles offset onto the fuser member may subsequently transfer to other parts of the machine or onto the support in subsequent copying cycles, thus increasing the background or interfering with the material being copied. The so called "hot offset" occurs when the temperature of the toner is raised to a point where the toner particles liquify and a splitting of the molten toner takes place during the fusing operation with a portion remaining on the fuser member. The hot offset temperature or degradation of the hot offset temperature is a measure of the release property of the fuser roll, and accordingly it is desired to provide a fusing surface which has a low surface energy to provide the necessary release. While many materials may initially function with good release properties with continued use, they tend to be contaminated with paper fibers, debris and toner as a result of hot offset of toner, thereby increasing the surface energy of the roll and perpetuating the destruction of release performance. In addition, once the roll becomes contaminated the hot offset temperature starts to reduce and may reach a level near or below the minimum temperature necessary to fuse the toner image thereby leading to both incomplete fusing of the toner image and offsetting of the toner image to the fuser roll. Once the fuser roll begins to be contaminated, contaminates are likely to be transferred to the pressure roll because it is generally of higher surface energy material.
It has also been proposed to provide toner release agents such as silicone oil, in particular, polydimethyl silicone oil, which is applied on the fuser roll to a thickness of the order of about 1 micron to act as a toner release material. These materials possess a relatively low surface energy and are suitable for use in the heated fuser roll environment. In practice, a thin layer of silicone oil is applied to the surface of the heated roll to form an interface between the roll surface and the toner image carried on the support material. Thus, a low surface energy, easily parted layer is presented to the toners that pass through the fuser nip and thereby prevents toner from offsetting to the fuser roll surface.
In addition, further difficulties may be encountered in such a fusing system in that the operational latitude may be unduly restricted. By operational latitude, it is intended to mean the difference in temperature between the minimum temperature required to fix the toner to the paper, the minimum fix temperature, and the temperature at which the hot toner will offset to the fuser roll, the hot offset temperature. Accordingly, as the hot offset temperature is reduced, this latitude is restricted. Typically, the fusing latitude must be maintained greater than 40.degree. F. and preferably of the order of 60.degree. F. to 70.degree. F. This is necessary for modern day reproducing flexibility which requires the capability to use many different types and weights of paper, different toner materials and amounts thereof as well as respond to use in a wide variety of speeds, and other operational conditions. It is also true that greater latitude is required to provide high quality copies particularly where toner pile height is increased to provide improved copy quality.
Typically fuser members such as the fuser roll have a thin elastomeric surface layer applied to a metallic cylindrical sleeve such as aluminum which is heated by a heating element disposed in the center of the aluminum sleeve. Since the fusing temperature is of the order of about 400.degree. F., the design of the fuser roll is such as to minimize power requirements to maintain the fusing temperature at the surface of the elastomer layer. Accordingly, the elastomer layer is generally designed to be as thin as possible and typically has substantial quantities of thermally conductive filler such as alumina, iron oxide and others added thereto. The thermally conductive filler increases the thermal conductivity of the elastomer layer thereby minimizing the thermal barrier to heat radiating from inside the fuser member to the outermost layer of the elastomer and reducing the power requirements. While the fillers such as alumina and silica are effective in increasing thermal conductivity of the elastomer layer since these particulate materials are relatively high surface energy materials when incorporated in the elastomer layer, the release properties of the elastomer layer are gradually degraded with continuing use. As a result, the hot offset temperature is reduced and the fusing latitude may also be reduced with time.
Accordingly, there is a continuing need for a fuser member having an elastomer surface fusing layer thereon of high thermal conductivity and which maintains good release characteristics on the elastomer surface.